Recently, there has been wide spread use of; wavelength division multiplexed (WDM, hereinafter) optical communication system, in which a single optical fiber is used both as a transmission line and as an optical to electrical converter (an O/E converter, hereinafter) for converting an optical signal into an electrical signal for use as a receiver. In the WDM optical communication system, since a single optical fiber can propagate plural optical signals with different wavelengths, lights of different wavelengths are respectively assigned to plural electrical to optical converters (E/O converters, hereinafter), multiplexed and transmitted through the single optical fiber in a transmitter, and the received optical signals are demultiplexed and respectively received by the plural O/E converters in a receiver. Hence the transmission capacity of the system can be remarkably increased. When a transmission distance between the E/O and O/E converters is long in the aforementioned system, the input optical signal levels of the O/E converters are decreased on account of the transmission loss of the optical fiber, and it becomes difficult to precisely receive optical information.
Accordingly, an optical amplification repeater is inserted at an intermediate point, where the WDM optical signals are directly amplified. In the long distance optical communication system, in which the plural optical amplification repeaters are used, the output powers of the optical amplification repeaters should be selected so that the transmission characteristic of the system is optimized in order to minimize the effects of nonlinearity of the optical fiber and noise caused by spontaneous emission (ASE) in the optical amplification repeaters.
The important components of the conventional optical amplification repeater are an Er-doped (erbium doped) optical fiber and a laser diode (LD, hereinafter) for pumping the same. The light emitted from the LD is supplied to an end of the Er-doped optical fiber and pumps the same. When the WDM optical signals are supplied at the other end of the pumped Er-doped optical fiber, the incident WDM optical signals are amplified therealong. The output power of the optical amplification repeater is monitored at its output port via an O/E converter, which is fed back to the control circuit of the LD. The LD for pumping the Er-doped optical fiber is so controlled that the total output power of the optical amplification repeater maintains a predetermined value.
As mentioned in the above, since the conventional optical amplification repeater is so controlled that its total output power is kept to be constant independently of the state of the input optical signals, a total output power of the optical amplification repeater is invariant, even in cases when a failure arises in the optical transmitter of the WDM optical communication system or transmission capacity is expanded by increasing the number of the optical signals.
Accordingly, as shown in FIG. 2, an output power of each of the optical signals is changed in accordance with the number of the optical signals, and the transmission characteristic degrades in case that the number of the optical signals is small, hence this number cannot be selected at will.